1. Field of the Invention
The present invention relates to a method for crystallizing polysilicon, and more particularly, to a method for crystallizing polysilicon using a laser with a ramp shaped cross sectional profile.
2. Discussion of the Related Art
Liquid crystal display devices are widely used because they are lightweight, thin, short, and small and have superior portability. As the demand for video display devices requiring high speed operation characteristics increases, the effort to develop video display devices capable of a high speed operation is advancing.
The development of high speed liquid crystal video display devices is actively in progress. A lot of effort is focused on the enhancement of the operation speed of switching devices that determine the operation characteristics of a liquid crystal display device.
A thin film transistor (TFT) is commonly used as the switching device in a liquid crystal display device. The operation speed of the TFT largely depends upon the performance of a silicon thin film constituting the channel.
Typically, amorphous silicon is used as a channel layer of the TFT. Amorphous silicon has a poor electric mobility. Therefore, the study of applying polysilicon having a high electric mobility of several tens to several hundreds of times higher than conventional amorphous silicon is underway.
Polysilicon has an electric mobility of up to 100 cm2/Vsec, and thus has a far superior operation speed versus amorphous silicon having an electric mobility of 0.1 to 0.2 cm2/Vsec.
One common method for forming polysilicon includes crystallizing an amorphous thin film. An example of crystallizing amorphous silicon includes heating amorphous silicon in a furnace. However, this method is problematic in that the crystallization speed is low. Further, a glass substrate may be used as a substrate of a liquid crystal display device, and because the glass substrate is deformed at a temperature higher than 600° C., the heating method in which crystallization is carried out at a temperature higher than 600° C. will not work to crystallize amorphous silicon utilizing glass as a substrate.
To overcome this problem, a laser crystallization method has been devised. The laser crystallization method induces crystallization by instantaneously melting an amorphous silicon layer by irradiating a high intensity laser on a small region and then cooling it.
The laser crystallization method allows crystallization at a temperature lower than a glass transition temperature. The crystallization of amorphous silicon using a laser crystallization method will be described briefly with reference to FIG. 1.
FIG. 1 is a graph showing the relation between laser energy density irradiated on amorphous silicon and the resulting size of the crystallized particles. As shown in FIG. 1, the crystallization of amorphous silicon may be divided into a first region, a second region, and a third region depending upon the intensity of the incident laser energy.
The first region is a partial melting region, where the intensity of the laser energy irradiated onto the amorphous silicon layer melts only the surface of the amorphous silicon layer. After irradiation, the surface of the amorphous silicon layer is partially melted in the first region, whereby small crystal grains form on the surface of the amorphous silicon layer after a solidification process.
The second region is a near-to-complete melting region, where the intensity of the laser energy, being higher than that of the first region, almost completely melts the amorphous silicon. After almost complete melting, the remaining nuclei are used as seeds for a crystal growth, thereby forming crystal particles with an increased crystal growth as compared to the first region. However, the crystal particles formed in the second region are not uniform. The second region is also narrower than the first region.
The third region is the complete melting region, whereby laser energy with an increased intensity, as compared to that of the second region, is irradiated to completely melt the amorphous silicon layer. After the complete melting of the amorphous silicon layer, a solidification process is carried out through a cooling procedure, so as to allow a homogenous nucleation, thereby forming a crystal silicon layer formed of fine and uniform crystal particles.
In this method of fabricating polysilicon, the number of laser beam irradiations and the degree of overlap are controlled so as to form uniform large and rough crystal particles by using the energy density of the second region.
A crystallization method in which the excimer laser is commonly used as a light source and the intensity of the laser energy is within the second region of FIG. 1 is referred to as excimer laser annealing (ELA).
In the crystallization processing using the ELA method, because a strong laser energy is irradiated directly on the surface of an amorphous silicon film but a relatively weak laser energy is irradiated on a lower part of the amorphous silicon film, the surface completely melts, but the lower part does not completely melt. Therefore, the silicon that does not melt acts as seeds and crystallization starts around the seeds; thus crystals of large and small size are formed.
On the other hand, if the intensity of a laser energy irradiated on the silicon layer reaches the third region, the amorphous silicon in an irradiated region is all melted, and no nucleus around which crystals can grow exists.
Thereafter, nuclei are randomly formed in the amorphous silicon as it cools, and crystals grow around the nuclei. As a result, the grains formed are very small in shape.
However, if crystallization is performed by using a laser in the complete melting region and utilizing a laser mask having a given size, the amorphous silicon layer that is not melted act as seeds, and crystallization grows laterally. When a laser irradiates an amorphous silicon layer by using a laser mask with an opening, the amorphous silicon is cooled by both lateral sides, i.e., the amorphous silicon layer which is not irradiated by the laser immediately after the laser irradiation finishes because the solid amorphous silicon layer at the sides has a higher thermal conductivity than the insulating layer under the amorphous silicon layer.
Therefore, the amorphous silicon melted by the laser energy undergoes a grain growth using the unmelted amorphous silicon at its sides as nuclei. At this time, grains are crystallized in a regular pattern in a lateral direction. As a result, the crystalline material that grows laterally produces one grain boundary in the center to form a large grain.
The above crystallization method is called as a sequential lateral solidification (SLS) because the grain grows laterally. The size of a laterally grown grain is usually 1-2 μm.
While the size of a grain resulting from a general laser annealing is several tens of mm, the size of a grain obtained through the above SLS crystallization method is several μm. Thus, polycrystalline silicon having the above grain size is applicable to a switching device where a large electric mobility can be realized.
When taking the maximum size of a grain that can grow by the sequential lateral solidification into account, if the sequential lateral solidification is performed at both lateral sides, it is possible to obtain a crystalline body in which a grain has only one grain boundary, and the crystal size is larger.
FIG. 2 shows a SLS crystallized silicon which is crystallized laterally at lateral sides and meets at one grain boundary formed in the center.
However, because of the development of integral type liquid crystal display devices which are on one substrate with driving elements as well as pixel elements of the liquid crystal display device, a demand for the fabrication of polysilicon having faster operation characteristics results.